How to put Mandarin Chinese on a website?

I have a client who wants me to create a Mandarin Chinese version of the site for him.

Is there anything special I have to do in order to show this language on a website?

You need to specify a font that includes Chinese ideographs.
You need to use a character encoding that can represent those characters (preferably UTF-8).
You should specify the language using the lang attribute. Do it for the root element if the whole page is in Mandarin, or for a specific container if only part of the page is in Chinese.
For HTML:

<html lang="zh">

For real XHTML:

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">

For pretend-XHTML (served as text/html):

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh" lang="zh">